1. Field of the Invention
The invention relates generally to the field of well logging. More particularly, the invention relates to improved techniques for maintaining positive contact between logging instruments and the borehole wall.
2. Background Art
Various well logging techniques are known in the field of hydrocarbon exploration and production. A significant number of these techniques require continuous positive contact between the logging instrument and the borehole wall in order to acquire high quality data required for accurate formation evaluation.
Numerous wireline tools are known in the art that can provide adequate pad contact with the borehole wall. These technologies typically use articulated sondes, springs, or hydraulic calipers to force pads that are equipped with logging sensors to move outward from the tool body to make contact with borehole walls.
For example, U.S. Pat. No. 4,468,623 issued to Gianzero et al. discloses an earth formation investigating tool that can detect borehole wall features with millimeter resolutions. The tool uses articulated arms to push electrodes into contacts with the wall of the borehole. The electrodes measure the conductivities of the borehole wall as the tool is pulled up the borehole. U.S. Pat. No. 4,567,759 issued to Ekstrom et al. discloses a method and apparatus for producing high resolution image from the data collected by tool described in the Gianzero patent. The apparatus and techniques disclosed in these two patents are known in the art as formation micro imager and sold under the trade name of FMI™ by Schlumberger Technology Corporation (Houston, Tex.). Other wireline tools that can provide high-resolution image of the formation using sensors in contact with the borehole wall include that disclosed in U.S. Pat. No. 6,191,588 B1 issued to Chen.
Many MWD and LWD tools are known that measure various formation and drilling parameters. Most of these tools have sensors mounted on metallic stabilizer-like pads that can withstand the harsh environments while drilling. The pads and the sensors often do not contact the wall of the borehole. One example of these tools is the Resistivity-at-the-Bit (RAB) tool described in U.S. Pat. No. 5,235,285 issued to Clark et al. and U.S. Pat. No. 5,339,037 issued to Bonner et al. On the other hand, the use of contact devices in MWD and LWD tools is limited. MWD or LWD tools that can make contact with the borehole wall typically use simple mechanical protruding devices. These protruding devices may not be able to maintain positive contact with the borehole with varying diameters (due to washouts etc.) or in wells with atypically large diameter boreholes. In addition, deviated wells and sharp doglegs (short radius of curvature bends in the well) can cause eccentering of MWD and LWD tools, leading to loss of contact with a significant portion of the borehole wall. In some situations, the protruding devices may cause problems with respect to the tool trajectory. It is known that the lower portion of a drill string may have a reverse angular velocity relative to the upper portion of the drill string. The reverse velocity may cause the drill string to lock up or may destroy the drilling assembly altogether.
Other LWD or MWD devices maintain borehole contact by keeping a section of the tool housing (or sleeve) stationary in the well, while allowing the drilling assembly located below the sensors to rotate. The section that is kept stationary may include sensors for measuring formation properties while the section is in contact with the borehole wall. One example of such logging tools is disclosed in the published U.S. patent application Ser. No. 2002/0062992 A1 by Fredericks et al. This approach is similar to drill bit steering technologies. The sensors on the stationary section cannot provide 360-degree coverage around the borehole, and the approach is impractical for covering a long stretch of the borehole. Furthermore, tool sticking may occur due to the prolonged stationary contact between the tool and the borehole wall, resulting in expensive and time-consuming fishing operations and/or possible loss of the tool and drilling assembly.
It is therefore desirable to have improved apparatus and methods that can enable MWD and LWD tools to establish and maintain contacts with the borehole wall in wellbore regardless of tool trajectory, borehole diameters, deviation, rugosity, or wash out.